For heavy duty material handling vehicles, it is known to provide suspension systems utilizing hydraulic support cylinders, either in an independent or semi-independent mounting arrangement. The objective is to provide a cushioned ride during the traveling mode of operation through the use of the cylinders and a stabilized suspension for the vehicle during the working mode of operation. For the traveling mode, it is known to provide pneumatic/hydraulic accumulators to provide a cushioning effect. Typically, the cylinders are coupled with transfer lines for moving the hydraulic oil between the extend and retract sides of the individual cylinders. It is also common to trap oil in the extend sides of the cylinder in order to provide a stabilized vehicle platform during the working mode of operation, such as during lifting a load by a crane or the use of a bucket on an excavator to dig.
One of the earliest examples of the suspension system of the type described is found in the Peiffer U.S. Pat. No. 3,550,993, issued Dec. 29, 1970. In this arrangement, the hydraulic circuit is capable of trapping fluid in the cylinders on the extend side for working. During the travel mode, fluid is exchanged between the extend and retract sides of each individual cylinder. However, a true oscillating motion is not possible since each pair of cylinders for either the front or rear axles are not coupled together. With such an arrangement, maintaining a level positioning of the vehicle is more difficult, and to compensate for this shortcoming, the use of level regulation valves operating continuously to modify the supply of hydraulic oil to all four cylinders of both axles is required. The complexity of the suspension system of the type in the '993 patent leaves much to be desired.
A later approach set forth in U.S. Pat. No. 4,174,854, issued Nov. 20, 1979, provides an advance in that an oscillation of the front and/or rear axle is provided through a modified hydraulic circuit. However, as in the '993 patent, constant monitoring of the height and the attitude of the vehicle is required to operate level regulation valves. In order to provide a stabilized suspension system for the working mode of operation, the '854 patent teaches rigidly locking the frame to the axle. A similar arrangement is found in the U.S. Pat. No. 5,368,322 issued Nov. 29, 1994, and this patent is one of the first to teach the use of a 4-bar assembly in the suspension to limit lateral axle movement in a hydraulic cylinder suspension system.
In more recent years, there have been several hydraulic circuits for automotive suspension systems designed to work as helpers for a basic mechanical spring system, such as U.S. Pat. No. 5,449,194, issued Sep. 11, 1995. These patents follow a reasonably long line of developments in this particular field and are directed in large part to combined systems, such as for working in concert with the brakes of the vehicle. For a proper operation, an even more complex system, such as with multiple pneumatic supply reservoirs and control valves, are required.
In each of the above described suspension systems, the nature of the operation requires extra complexity in terms of the mechanical structure, as well as the hydraulic and pneumatic circuits to control the support cylinders. Furthermore, even with the more complex circuitry, the operation of the system has been found to need improvement. In particular, there has been little progress in the development of a suspension system for the rear axle of a heavy duty vehicle that matches a superior cushioned ride in one mode, with a more stabilized working platform in the other mode. An arrangement that provides a more cushioned ride in the traveling mode of operation is needed in order to relieve the stress and fatigue of the operator. This system should also minimize the mechanical stress on the frame and the other vulnerable components of the vehicle. It would be desirable to maintain a simulated oscillating movement. This allows elimination of the center pin, bolster and the reinforced axle frame. The center of the vehicle would be opened with free space. Not only would there be a considerable initial cost savings and significant weight savings, but also better access to the engine of the vehicle and other components would result. Thus, the need for the present improved suspension system combinating these changes is apparent.